Cooling water systems which circulate cooling water for cooling steam discharged from a steam turbine are provided in thermal power plants in some cases. The cooling water system includes a condenser which cools steam and a cooling tower which cools cooling water heated in the condenser. There is a need to constantly supply makeup water to the cooling water system because cooling water is evaporated in the atmosphere in the cooling tower. Substances which cause deterioration of the cooling water system can be included in the makeup water. Examples of the substances which cause deterioration include salts which can cause corrosion or be converted into evaporation residues and silica which can cause scale. A certain amount of cooling water is discharged, and a certain amount of chemicals are injected not to deteriorate the cooling water system.
Patent Literature 1 discloses a method of blowing (discharging) circulating cooling water and injecting makeup water when electrical conductivity of the cooling water is higher than a set value, and stopping the blowing when the electrical conductivity of the cooling water is lower than the set value, as a method of controlling chemical injection in conjunction with an amount of makeup water.